The present invention relates to a driving mechanism for a type drum in an impact printer.
FIG. 1 shows a conventional driving mechanism for a type drum in an impact printer. A type drum 10 has a plurality of type characters circumferentially arranged for each column and is secured to a type drum shaft 11 which is rotatably supported in side plates 12 and 13. A plurality of hammers 15 are disposed adjacent the type drum 10 arranged in the axial direction thereof in the well known manner. A type drum gear 14 is secured to the type drum shaft 11 at the outside of the plate 12. A first drive gear 18 driven by a motor (not shown) is engaged with a second drive gear 20 which is rotatably mounted on a shaft 22 secured to the plate 12. The gear 20 is engaged with the type drum gear 14 through an intermediate gear 21 which is rotatably mounted on a shaft 23 secured to the plate 12.
Thus, when the gear 20 is rotated in the direction of an arrow A, the type drum gear 14 is rotated in the direction of an arrow B through the intermediate gear 21 to rotate the type drum 10. During the rotation of the type drum 10, hammers 15 strike the type character to print the type on a paper 16 at a proper timing.
In such a printer, when all hammers 15 strike the type characters on the type drum 10 at the same time, the striking force of the hammers 15 operate as a brake against the type drum, which may cause the type drum 10 to stop or slow down. On the other hand, the gear 20 continues rotating so that small elastic deformation occurs in the gear train comprising gears 18, 20, 21 and 14. Immediately after that, hammers 15 are retracted to release the drum. Accordingly, the type drum 10 is rotated in the direction of the arrow B at a higher speed than the rated speed by the returning of the deformation. However, when the type drum 10 rotates an angle corresponding to the sum of backlashes in the gear train, the drum is stopped by the rotary shaft of the motor which rotates at a constant speed. The type drum bounds back and the movement repeats. Accordingly, the type drum 10 pulsates, rendering printed letters out of alignment which means decrease of the quality of printing.